1. Field of the Invention
The present invention relates to a trap circuit for eliminating a specific frequency signal included in an input signal, and more particularly, it relates to a trap circuit which is formed with a plurality of cascade-connected three-terminal ceramic resonators.
2. Description of the Background Art
In order to prevent a picture fault caused by a sound signal, a sound signal trap circuit corresponding to its frequency is provided in a picture signal circuit for a television receiver.
FIG. 4 shows an exemplary trap circuit T.sub.1, which is included in a television receiver, provided on a front stage of a picture-amplification circuit 13. A picture signal supplied from a picture circuit is received through an input terminal 10 of the trap circuit T.sub.1, and inputted to the picture-amplification circuit 13 through a series circuit which is defined by a resistor 11 and an inductor 12. Input and output terminals of a three-terminal piezoelectric ceramic resonator 14 are connected to both ends of the inductor 12, while a common terminal of the resonator 14 is grounded. A resistor 15 is connected between the output terminal of the resonator 14 and the ground. An output end of the picture-amplification circuit 13 is connected to a color demodulator circuit 16, whose output end is connected to a cathode-ray tube 17.
When such a trap circuit T.sub.1 is formed with a single inductor 12 and a single resonator 14 in this manner, its attenuation amount is extremely reduced in ranges outside the resonance frequency f.sub.0 of the resonator 14, at which large attenuation amount is attained, as shown in FIG. 5. If such a trap circuit T.sub.1 employs a resonator with resonance frequency of f.sub.0 =4.5 MHz, for example, it is impossible to significantly attenuate a disturbing signal caused by FM signals such as the "Educational FM" signal, which is in a frequency domain higher than 4.5 MHz, level, although a sound signal of 4.5 MHz can be effectively eliminated. (By the "Educational FM" is meant the FM frequency band of 88.1-91.9 MHz, to be reserved for non-profit educational programs in Japan according to a recommended design guideline by the Electronic Industries Association. On TV receivers, this appears as disturbing signals at 4.85 MHz and thereabove with intervals of 0.35 MHz. Even if trap resonators corresponding to these frequencies are provided, they alone cannot attenuate near-by disturbing signals to the level of about 20 dB.) Although it is possible to construct a trap circuit for each disturbing signal, a large number of parts as well as a large substrate are required for such a circuit.
In order to attenuate a plurality of disturbing signals by a single trap circuit, it may be attempted, as shown in FIG. 6 as a hypothetical example, to form a trap circuit T.sub.2 by connecting between an input terminal (IN) and an output terminal (OUT) a plurality of inductors L.sub.1, L.sub.2 and L.sub.3 in series with a plurality of resistors R.sub.1, R.sub.2 and R.sub.3, and connecting resonators t.sub.1, t.sub.2 and t.sub.3 having different resonance frequencies in parallel with the respective inductors. In FIG. 6, R.sub.a and R.sub.b denote parallel resistors, and GND indicates ground terminals. In this hypothetical example, large attenuation can be attained at resonance frequencies f.sub.1, f.sub.2 and f.sub.3 of the resonators t.sub.1, t.sub.2 and t.sub.3, as shown in FIG. 7.
The circuit constants of such a trap circuit T.sub.2 may, for example, be as follows:
f.sub.1 : 4.5 MHz PA1 f.sub.2 : 4.85 MHz PA1 f.sub.3 : 5.42 MHz PA1 I.sub.m1 : 280 ohms PA1 I.sub.m2 : 240 ohms PA1 I.sub.m3 : 245 ohms PA1 L.sub.1 to L.sub.3 : 10 microhenries each PA1 R.sub.a : 50 ohms PA1 R.sub.b : 560 ohms PA1 R.sub.1 : 150 ohms PA1 R.sub.2 : 47 ohms PA1 R.sub.3 : 560 ohms
where I.sub.m1 to I.sub.m3 represent impedance across input and output terminals of the resonators t.sub.1 to t.sub.3 and the ground.
When the inductors and the resonators are cascade-connected in this manner, however, the attenuation is reduced by peaking at intermediate regions between the resonance frequencies f.sub.1, f.sub.2 and f.sub.3, and hence it is impossible to continuously attain attenuation of about 20 dB, which is generally required for effectively eliminating or suppressing a disturbing signal caused by FM signals such as "Educational FM" signal. Thus, it has been impossible to simultaneously eliminate a disturbing signal having a specific frequency f.sub.1 and those in intermediate regions between the resonance frequencies f.sub.2 and f.sub.3.